The limb can be thought of as three or more segments: the humerus or femur (stylopodium), the radius/ulna or tibia/fibula (zeugopodium), and the manus/pes (autopodium). In some groups, the scapula and pelvis are added as additional segments, and in some groups the digits are elongated to become segments in their own right. The number of segments and their proportional length vary considerably in animals with different locomotor styles. Proportionally long distal segments increase the gear ratio of the limb and are typical of cursors, proportionally long proximal segments decrease the gear ratio and are typical of fossorial, natatorial, and graviportal specialists.

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Measure the limb segments in the ten species by measuring the total length of the humerus, radius, and third metacarpal in the fore limb and the femur, tibia and third metacarpal in the hind limb. Convert the measurements to percentages of limb length (the sum of the three lengths) and plot them on the ternary diagrams below. Indicate the locomotor style of each point.

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Humerus length (%)

To what extent do the plots separate the locomotor categories? Do the fore limb and hind limb proportions separate the species by their locomotion? Do some locomotor styles have stronger parallels between fore lime and hind limb specialization than do others?

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2.

Lever mechanics

Calculate the gear ratio of the ulna in the ten species by measuring their in-lever and outlever lengths. The in-lever is the distance from the end of the olecranon process (the elbow) where the triceps muscle inserts to the middle of the semilunar notch, where the radius and ulna pivot around the condyle of the humerus. The out-lever is the distance from that point to the distal end of the ulna. Which species will have the highest velocity of movement at the wrist if equal velocities are applied at the olecranon?

Didelphis virginiana

Ambulatory

Procyon lotor

Ambulatory

Lontra canadensis

Aquatic

Phoca vitulina

Aquatic

Gazella thompsonii

Cursorial

Vulpes vulpes

Cursorial

Macropus robustus

Saltatorial

Sylvilagus floridanus

Saltatorial

Choloepus didactylus

Scansorial

Sciurus niger

Scansorial

Gear ratio

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Pronation, supination and the radial head

Pronation and supination mean turning the manus palm down and palm up respectively. Some animals, like cats and humans, can easily invert their hand this way, others, like dogs and deer, cannot. Pronation and supination are advantageous for climbing, manipulating food, and digging, but they are disadvantageous for high-speed locomotion or bearing heavy weight. One structural feature that is associated with pronation and supination is the shape of the radial head, the cup-shaped depression at the proximal end of the radius: the rounder the head, the better the ability to pronate and supinate. Diagram the shape of the radial head in three species, one cursorial, one scansorial, and one of your choice. Which species probably have the strongest pronation/supination ability based on this feature?

Diagram showing how the teres process of the scapula, the olecranon process of the ulna and the calcaneal process of the ankle can change the gear ratio. (from Hildebrand, Analysis of Vertebrate Structure).

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Diagram showing how extensions at several joints in the fore limb sum to give the total extension. Adding limb segments and lengthening them produces a longer total extension length. Lengthening the distal segments relative to the proximal ones increases both extension length and the mechanical advantage (gear ratio) of the limb. (from Hildebrand, Analysis of Vertebrate Structure)

Ambulatory locomotion Ambulatory locomotion is a generalized form of terrestrial locomotion that allows for walking, running, climbing, and digging but is specialized for none. Ambulatory species tend to have combinations of characteristics from other locomotor types.

Scansorial/Arboreal locomotion 1. 2. 3. 4. 5. 6. 7. 8.

Low gear ratio in some joints, high gear ratio in others Ability to pronate and supinate (invert and evert) manus and pes Elongate wrists and ankles for springing Proportionally short olecranon process on ulna in hanging/swinging specialists, proportionally long olecranon in scrambling/springing specialists Elongated tarsals in springing specialists, parallels saltators Well-developed pronation and supination ability in the fore limb and often the hind limb Flexibility in wrist and ankle, tarsals configured so that the pes can be inverted Grasping pollux and hallux

Features of the limbs of a sloth, an arboreal species that suspends itself from limbs. (from Hildebrand, Analysis of Vertebrate Structure).

Cursorial locomotion 1. 2. 3. 4. 5. 6.

Long stride (digitigrade or unguligrade) High rate of stride High gear ratio Extension of the back (except in large animals tending toward graviportal) Hinge-like joints for stability Reduced digits for light distal limbs and stability

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(Polly, 2007, Mammal Limbs)

Comparison ot the radius and ulna of an aboreal kinkajou (left) with a cursorial pronghorn antelope (right). (from Hildebrand, Analysis of Vertebrate Structure).

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Tarsus and pes of cursorial animals. Note digit reduction and, in the mammals, the deeply grooved astragalus that limits movement to a single parasagittal plane. (From Hildebrand, Analysis of Vertebrate Structure).

Hinged joints between metatarsals and proximal phlanges in the Vicuna (from Hildebrand, Analysis of Vertebrate Structure).

Comparison of the distal humerus (top) and carpus (bottom) of saltators (left) and arboreal species (right). (from Hildebrand, Analysis of Vertebrate Structure).

Fossorial locomotion 1. 2. 3. 4. 5.

Plantigrade Low gear ratio Specialized claws Large acromion and teres processes on scapula Large deltoid crest on proximal humerus and large lateral condyle and supinator ridge (the point of origin for muscles that invert or supinate the manus, or hand) on distal humerus 6. Proportionally long olecranon process on ulna

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Features of the fore limb of fossorial mammals. (from Hildebrand, Analysis of Vertebrate Structure).

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Aquatic/Natatorial locomotion

Features of the fore limb of aquatic mammals. (from Hildebrand, Analysis of Vertebrate Structure).